The present invention relates to a process and to an apparatus for detecting the presence of a physical phenomenon. This process and this apparatus are for example applicable to the detection of nuisances such as the presence of a pollutant in the air. It is also possible to envisage the use of this process and apparatus for detecting combustible vapours such as hydrocarbon vapours floating on water. This process and apparatus may also be used for determining the hygrometry of the air, for detecting thunderstorms, for detecting fire damp in mines, for detecting the presence of a colour, for checking surface states, etc.
It is known that the detection of a physical phenomenon is of particular interest in the study of nuisances. Thus, the discomfort which may result from a disturbing or perturbing agent can be just as great in the case of rapid variations of a low content of said agent and in the case of a high constant content. The perception of odours for example is linked to the level of the concentrations of these vapours in the air. The continuous measurement of the concentrations reveals very rapid variations of the instantaneous values. The amplitude of these variations is dependent on the heterogeneity of the pollutant-air mixture swept along by the wind. For a stationary observer, the variations in the concentrations of pollutant vapours occur when the pollution front arrives and may be reproduced as a result of the displacement of the plume formed by the vapours. A plume is never homogeneous and an observer will only observe a fixed vapour concentration if he remains on a single air flow line and if the pollutant source has a constant flow rate. However, this case is very rare, because air has a natural turbulence and consequently there is a certain spatial distribution of the pollutant in the air. The heterogeneity of the pollutant-air mixture consequently leads to a polluting turbulence, whose appearance is called "emergence" of the pollutant.
For measuring the pollution of a site, it is known to use an apparatus comprising a detector sensitive to the phenomena to be studied, means for amplifying and sampling the signals supplied by the detector and means for comparing the sampled signal with a threshold value. This sampling makes it possible to determine if for a given period the amplified signal from the detection means has exceeded a threshold value beyond which it is almost certain that a phenomenon exists.
The detectivity of such an apparatus is low in the case where the phenomenon is only slightly perceptible. This detectivity is limited on the one hand by the drift of the apparatus and on the other by the amplitude of the background noise resulting from the experimental conditions. Thus, under severe experimental conditions, certain very sensitive apparatus cannot be used. These problems can be obviated, but only as a result of making the equipment excessively complicated and in particular by realising a frequency spectral analysis of the signal representative of the phenomenon in order to reveal certain perturbations essentially due to the background noise superimposed on the signal which is representative of the phenomenon in question.